Fuel injectors for injecting gasoline fuel into an internal combustion engine are well known in the art (see U.S. Pat. No. 4,700,891). Such fuel injectors create fine atomization of liquid fuel, which improves the efficiency of the burning cycle.
Prior art gasoline injectors can be classified into the two categories of "top feed" or "bottom feed" injectors, describing the location where supply fuel enters the injector. In both designs, metered fuel exits the injector through the bottom. Metering is accomplished by a valve and seat in conjunction with a metering orifice. Upon energization of a magnetic coil surrounding the valve, the resulting magnetic force pulls the valve away from the seat allowing fuel to pass out through the metering orifice. A spring is typically incorporated to return the valve to its closed position when the coil is de-energized. Valve/seat configurations typically employed include a ball with a mating spherical seat or a valve and seat with mating conical surfaces. From the metering section, fuel flows to a spray-generating element which creates a spray pattern of fine droplets of fuel varying from a narrow stream to cones of different angles.
A solenoid assembly electromagnetically actuates the fuel metering valve. In the most common configuration, a coil consisting of a specified number of turns is wound around a bobbin. The magnetic field produced by the energized coil generates flux, which travels in a closed loop around the bobbin. An armature and a pole piece located just above the armature are positioned within the coil. The armature is the moving element which controls fuel flow by opening and closing the valve. The pole piece is the non-moving element which attracts the armature when the coil is energized. A fixed gap between the armature and pole piece prevents contact between them to minimize residual magnetism effects which would result in delayed closing of the valve. A non-magnetic stop or spacer is typically employed to maintain a fixed clearance when the valve is fully opened.
While fuel injectors for injecting liquid fuel, such as gasoline or diesel fuel, are common in the art, little has been done in the art with respect to fuel injectors for injecting liquified petroleum gas ("LPG") into an internal combustion engine.
Interest in alternative fuels, such as LPG, has increased in recent years due to the inherent cost and environmental advantages over other fuels. LPG has particularly received much attention as an alternative to gasoline or diesel fuel for use in internal combustion engines. Propane, the primary constituent of LPG, is a byproduct of the refining of gasoline, and it is a byproduct of the transfer of natural gases in pipelines. It is readily available and at costs far below that of gasoline.
LPG was recently listed under the Clean Air Act in the United States as a suggested alternative fuel because it is more environmentally compatible than gasoline. LPG burns more completely, producing less carbon monoxide and hydrocarbon emissions. Also, using LPG as a fuel reduces the emission of volatile organic compounds which occurs during gasoline refueling.
The U.S. Federal Government recently promulgated legislation, referred to as Corporate Average Fuel Efficiency (CAFE) standards, to promote the use of more environmentally compatible fuels. CAFE created a system of incentives which encourages manufacturers to build automobiles and trucks which use alternative fuels, such as LPG. As a result, there is increased interest in manufacturing and retrofitting automobiles and trucks to be fueled with LPG.
Although LPG in its gaseous form has been used as a reasonably effective fuel in internal combustion engines, there is an associated reduction in power capability as compared to liquid LPG fuels. This power reduction is mainly due to the reduced amount of air and fuel drawn into the intake manifold when the LPG enters the manifold in gaseous form.
With liquid LPG, a further gain in peak power (and simultaneous reduction in the emission of nitrous oxides) results from the cooling of air and fuel within the manifold from vaporization of injected LPG. This also reduces the tendency for engine knock.
Use of LPG in liquid form as a fuel is fairly new in the art. However, several obstacles are associated with attempting to inject liquid LPG directly into the intake manifold of an internal combustion engine. In particular, it is difficult to maintain LPG in its liquid state near the heated engine compartment. LPG has a very low boiling point (see FIG. 5 for the liquid-vapor phase boundaries for propane and isobutane, the primary constituents of LPG). Even under pressure, LPG will tend to bubble or boil as the boiling temperature at a given pressure is approached. The formation of bubbles, often called "champagning" or "flashing" causes inconsistent injection and poor air/fuel ratio control.
It is thus necessary to cool the LPG to prevent this bubbling or boiling. U.S. Pat. No. 4,489,700 discloses one approach, where cooling is accomplished by a spool tip in the injection nozzle which cools the LPG within the injector through heat transfer from the vaporization of injected LPG.
However, the device of U.S. Pat. No. 4,489,700 and other prior art fuel injectors have in large part been unsuccessful in maintaining the LPG injected into the intake manifold in a fully liquid state throughout the operating range of the engine.
Prior art LPG injectors have another disadvantage. They lack compatibility with existing gasoline internal combustion engine configurations. Prior art designs require significant changes in the power plant itself in order to convert an engine fueled with gasoline to one fueled with LPG. As a result, retrofitting existing gasoline-powered cars and trucks with a LPG fuel delivery system has not been a cost effective option.
Prior art LPG injectors are also not compatible with the existing engine control unit ("ECU") commonly used in gasoline-powered vehicles. The ECU monitors various engine parameters and controls injection by sending an electrical current which activates the fuel injector to cause fuel to be injected into the intake manifold of the engine. Prior art LPG injectors would require a dedicated system to control LPG injection. Thus, even though some software modification may be required, a LPG injector which is compatible with the existing ECU in gasoline-powered vehicles would further decrease the complexity and cost of retrofitting a gasoline vehicle with a LPG fuel supply system.
A significant problem encountered when injecting LPG is that it commonly contains considerable contaminants. These contaminants or "heavy ends" tend to collect in the injector over long periods of time. With prior art LPG injectors, the injector must typically be replaced once contaminants have collected in the injector in an amount sufficient to significantly affect the injector's performance. Thus, a fuel injector designed to allow removal of such contaminants without requiring replacement or substantial rebuilding of the injector would result in substantial cost savings.
Consequently, it is clear that a simple, low cost, efficient, and effective LPG fuel injector continues to be needed which accomplishes one or more of the following: maintains LPG in a fully liquid state during injection; is compatible for retrofit on vehicles having gasoline injection systems; allows use of the existing ECU common to gasoline-injected engines; and permits cleaning of contaminants without requiring substantial rebuilding of the injector.